The invention relates to a cutting tool (or cutting tap) used for the production of internal screw threads in a substrate. More specifically, the invention pertains to a cutting tool (or cutting tap) that has the spiral (helical) flute geometry wherein the spiral flute cutting tap is made from cemented tungsten carbide tool materials.
Mechanisms and machine components that require screw threads have a long history in technology. Specifically, the application of screw threads as fastener components is the dominate way to join parts into assemblies. Although there are many ways to generate screw threads, including both internal screw threads as well as external screw threads, experience has shown that taps (or cutting taps) are the favored way to generate the internal screw thread.
There currently exist two basic tapping methods to generate internal screw threads. The dominant tapping method is by cutting material from the wall of the hole. The cut material is then removed to produce a helical V shaped screw thread. In the other basic tapping method, material is displaced to form an internal screw thread. The cutting method of forming an internal screw thread requires lower torque than does the displacement method.
The dimensional accuracy of the shape and size of the internal screw thread controls the precision and fit of the screw thread assembly. In addition, the speed of tapping, i.e., the production time that it takes to cut the internal screw thread, affects the overall cost to produce an internal screw thread.
The configuration and size of the internal screw thread to be produced dictates the geometry of the tap that may be used. In general, an internal threaded hole may be are either open at the bottom to be a through hole or the internal threaded hole may be closed at the bottom to be a blind hole.
Through threaded holes and blind threaded holes that have a larger diameter (e.g., generally equal to or greater than 12 millimeters in diameter) may be tapped (or cut) with a cutting tap that has a straight flutes. A straight fluted cutting tap is shown and described in PCT Patent Publication WO 2004/076108 A2 for a PRECISION CEMENTED CARBIDE THREADED TAP to Henderer et al. A straight fluted cutting tap may be used to cut through threaded holes or blind threaded holes that have a smaller diameter (e.g., generally less than 12 millimeters in diameter) in materials that form a short, broken chip (e.g., cast iron or aluminum). However, a straight fluted cutting tap is not effective to cut a through threaded hole or a blind threaded hole that have a smaller diameter in materials that produce a continuous chip (e.g., ductile steels such as, for example, AISI 4340 steel). In such a case, the continuous chips generated from the cutting operation bind in the flute of the straight fluted cutting tap and cause the cutting tap to break or otherwise not function at an acceptable level.
There are two generally used remedies for this situation where continuous chips generated from the cutting operation bind in the flute of the straight fluted cutting tap and cause the cutting tap to break or other wise not function at an acceptable level. In the case of cutting through threaded holes, there has been used a spiral pointed cutting tap or a left hand spiral fluted cutting tap (when cutting a right hand thread helix) push the chip ahead of the cutting tap thereby avoiding the chip from binding or jamming in the flute of the straight fluted cutting tap. In the case of cutting blind holes, there has been used a right-hand spiral fluted cutting tap which pulls the chips out of the threaded hole thereby preventing the chips from binding or jamming in the flute of the straight fluted cutting tap.
At the present time, spiral fluted taps (also called helical fluted taps) made from high-speed steel are effectively used to cut blind threaded holes. Specifications for high-speed steel (or high-speed tool steel) are set forth in ASTM Standard A 600 REV A entitled Standard Specification for Tool Steel High Speed.
Although high-speed steel spiral fluted cutting taps operate to produce satisfactory results, cemented tungsten carbide (e.g., cobalt-cemented tungsten carbide) is a preferred material over high-speed steels for manufacturing a cutting tool such as a spiral fluted cutting tap. This preference is due to the cemented tungsten carbide material possessing more optimum properties than high-speed steel such as higher hardness and high temperature stability including the ability to retain hardness at high temperatures. Typically, cutting tools manufactured from cemented carbide (e.g., cobalt-cemented tungsten carbide) can be used at cutting speeds that are at least three times higher than cutting tools manufactured from “high-speed” steel. In addition, cutting tools manufactured from cemented carbide (e.g., cobalt-cemented tungsten carbide) typically exhibit a useful tool life that is longer than the useful tool life of a cutting tool manufactured from “high-speed” steel.
Cemented carbide cutting taps with straight flutes, spiral points and left-hand spiral flutes have been developed and used at high speeds to cut threaded holes. However, cemented carbide right-hand spiral flute cutting taps that have the flute designs are like those commonly used for high-speed steel cutting taps experience chipping when tapping small diameter threaded holes. These right-hand spiral fluted cutting taps present cutting edges and edges at the heel of the flute that are prone to chipping due to the small included angles at the intersection of the flutes and thread flanks at these locations. Chipping results in breakage of the cutting tap or (at a minimum) an inability of the cutting tap to function at an optimum level.
There exists in patent literature different concepts employed for spiral (helical) fluted cutting taps. In this regard, U.S. Patent Application Publication No. 2004/0247406 A1 to Malagino describes a helically fluted tap with steam temperature flute surfaces and PVD coated thread surfaces. U.S. Patent Application Publication No. 2003/0138302 A1 to Newmark describes a helical fluted tap incorporated with a device that chamfers the hole. U.S. Patent Application Publication No. 2003/0118411 A1 to Flynn describes a spiral fluted tap with a helix angle that varies along the length of the tap. British Patent No. 700,843 to Haycock describes a spiral flute tap that has been ground such that the leading end face of each land is axially relieved or backed off at an angle. PCT Patent Publication WO 02/087813 A1 to Oknestam describes helical fluted taps with chip breakers incorporated with the surfaces of the flutes.
WIPO 02/28578 A3 (Apr. 11, 2002) to George describes a spiral fluted drill with a flute shape having a heel with a convex radius. However, there are substantial differences in the cutting action of taps and drills. In generating a hole, drills are required to displace and remove material from the center line to the wall of the hole, whereas taps remove material only from the wall of the hole. Since the direction of chip flow differs between the two tools, the required shape of the flute differs.
Japanese Patent Absract 06179121A (Dec. 14, 1992) to Haruo describes a spiral flute tap with a negative axial rake angle. Japanese Patent Abstract 04075816A (Mar. 10, 1992) to Haruo describes a spiral fluted tap that has the corner of the thread flanks and entry lead surfaces chamfered. Japanese Patent Abstract 01171725A (Jul. 6, 1989) to Haruo describes a spiral fluted tap with a chip curler groove.
In reference to the cutting taps disclosed in the above patent documents, none of these documents addresses the chipping problems that occur when highly wear resistant, but lower strength substrates (e.g., tungsten carbide or cemented carbides like cobalt-cemented tungsten carbide) are used for spiral fluted cutting taps in the cutting of threaded holes. This is especially the case for cutting taps used to cut threaded holes (through holes and blind holes) of a smaller diameter. Thus, it would be highly desirable to provide a spiral fluted cutting tap made from highly wear resistant, but lower strength substrates (e.g., tungsten carbide or cemented carbides like cobalt-cemented tungsten carbide) that can be used to cut threaded holes, and especially threaded holes that are of a smaller diameter (e.g., generally less than 12 millimeters).
Further, it would be highly desirable to provide a spiral fluted cutting tap that provides for improved accuracy in cutting the threaded hole, and especially in the cutting of blind threaded holes. This would be the case for spiral fluted cutting tap s made from cemented carbides or high-speed steels.
It would also be highly desirable to provide a spiral fluted cutting tap that provides for improved useful tool life in cutting the threaded hole, and especially in the cutting of blind threaded holes. This would be the case for spiral fluted cutting taps made from cemented carbides or high-speed steels.
It would also be highly desirable to provide a spiral fluted cutting tap that provides for improved speed in cutting the threaded hole, and especially in the cutting of blind threaded holes. This would be the case for spiral fluted cutting tap s made from cemented carbides or high-speed steels.